Asymmetrical Dependence of {Ba2+}:{SO4 2-} on BaSO4 Crystal Nucleation and Growth in Aqueous Solutions: A Dynamic Light Scattering Study

The impact of solution stoichiometry, upon formation of BaSO₄ crystals in 0.02 M NaCl suspensions, on the development of particle size was investigated using dynamic light scattering (DLS). Measurements were performed on a set of suspensions prepared with predefined initial supersaturation, based on the quotient of the constituent ion activity product {Ba²⁺}{SO₄ ^2-} over the solubility product K _sp (Ω_barite = {Ba²⁺}{SO₄ ^2-}/K _sp = 100, 500, or 1000-11,000 in steps of 1000), and ion activity solution stoichiometries (r _aq = {Ba²⁺}:{SO₄ ^2-} = 0.01, 0.1, 1, 10 and 100), at circumneutral pH of 5.5-6.0, and ambient temperature and pressure. DLS showed that for batch experiments, crystal formation with varying r _aq was best investigated at an initial Ω_barite of 1000 and using the forward detection angle. At this Ω_barite and set of r _aq, the average apparent hydrodynamic particle size of the largest population present in all suspensions increased from ∼200 to ∼700 nm within 10-15 min and was independently confirmed by transmission electron microscopy (TEM) imaging. Additional DLS measurements conducted at the same conditions in flow confirmed that the BaSO₄ formation kinetics were very fast for our specifically chosen conditions. The DLS flow measurements, monitoring the first minute of BaSO₄ formation, showed strong signs of aggregation of prenucleation clusters forming particles with a size in the range of 200-300 nm for every r _aq. The estimated initial bulk growth rates from batch DLS results show that BaSO₄ crystals formed fastest at near-stoichiometric conditions and more slowly at nonstoichiometric conditions. Moreover, at extreme SO₄-limiting conditions, barite formation was slower compared to Ba-limiting conditions. Our results show that DLS can be used to investigate nucleation and growth at carefully selected experimental and analytical conditions. The combined DLS and TEM results imply that BaSO₄ formation is influenced by solution stoichiometry and may aid to optimize antiscalant efficiency and regulate BaSO₄ (scale) formation processes.